296 Reply to Mr. Cooper's Queries. 



Pertenhall, Kimbolton, rather more than a mile under Mr. 

 C.'s reduced latitude. But to obtain the true latitude by an 

 altitude of the sun, the apparent time should be known to be 

 correct, and not exceed forty minutes before or after noon. 



I have myself been in the habit of taken altitudes of the sun 

 and stars with a sextant and an artificial horizon, for ascer- 

 taining the daily rate of ship chronometers and an astronomical 

 clock. For this purpose I have tried many fluids, as water, 

 oil, treacle, as circumstances have required, and find that 

 quicksilver strained through a piece of soft shammy leather, 

 and thus cleansed from dross and dust, is the best that can be 

 used for defining the limbs of the sun, &c. by direct and re- 

 flected vision. Yet by this process I despair of ever obtain- 

 ing the apparent time to the fraction of a second : if it can be 

 got to a second of time, I deem it an excellent observation, 

 considering the discrepancies there may be between the tabular 

 atmospherical refraction, after correcting it for pressure and 

 temperature, and the real refraction in the day-time, with the 

 probable error of the instruments after their adjustment. 



The method universally adopted by practical astronomers to 

 find the apparent time, and thence the error of a chronometer 

 or clock, when they are not provided with a transit instrument 

 fixed in the meridian, is by equal altitudes of the sun, or a 

 known fixed star; for by this method, the process of which 

 may be found in most treatises on navigation, neither the ac- 

 curacy of the latitude of the place, nor that of the declination 

 of the celestial object particularly depends, since the elements 

 are only necessary in taking out the equation of equal altitudes. 



With respect to double altitudes on shore, I have often 

 found the latitude by meridional altitudes of the sun and 

 known fixed stars, far more correct than by two altitudes taken 

 out of the meridian by the most accurate methods that have 

 yet been adopted. I therefore prefer a meridional altitude, 

 particularly of the sun as the largest object, for ascertaining 

 the latitude ; and equal altitudes for apparent time. 



Not the apparent, but the true altitude of the sun's centre 

 should be invariably used for finding the apparent time; and 

 thence the error of a chronometer or watch, by applying the 

 equation in its proper sign, after it is reduced to the time of 

 observation. 



The sun's right ascension as given in the Nautical Almanack, 

 is expressed in apparent solar time, not in sidereal time. 



The application of the quantities in Dr. Tiarks's tables al- 

 luded to by Mr. C, is for the stars only, as an easy method 

 of reducing sidereal into mean solar time. Had Dr. Tiarks 

 furnished you with two tables instead of his table No. 2, one 



for 



